The formation of the oxide ion, O²⁻(g), from oxygen atom requires first, and exothermic and then an endothermic reaction as shown below:
O(g) + e⁻ → O⁻(g) ; ΔfH⁻ = -141kJ mol⁻¹
O(g) + e⁻ → O²⁻(g) ; ΔfH⁻ = +780kJ mol⁻¹
Thus, process of formation of O²⁻ in gas phase is not favourable even though O²⁻ is isoelectronic with neon. It is due to the fact that:
Select one of the following: